1. Field of the Invention
The present invention generally pertains to apparatus for sorting fruit according to the color thereof, and more particularly, it pertains to apparatus for sorting individual fruit at relatively high speeds so as to make the apparatus adaptable for use in fruit packing house operations.
2. Description of the Prior Art
Fruits and vegetables have long been graded according to the surface color of the fruit which, in turn, relates to the quality of the fruit's interior. This has led to the visual grading of fruit by its color which, being dependent upon the grader's ability to perceive color differences, is influenced by working conditions and is degraded by fatigue. The desirability of electronic-mechanical means for sorting fruit according to color has long been recognized, and various systems, including those described hereinafter, have been proposed.
In quality sorting operations, apples are sorted according to shape, surface blemishes, point defects and degree of bruising as well as according to color. Using these criteria the apple industry presently sorts into four grades: extra fancy, fancy, "C" or commercial, and cull (in descending order of quality). However, most of this quality sorting effort is devoted to sorting as determined by estimating the percentage of apple surface that is the characteristic color of the apple being sorted, e.g., red in the case of Red Delicious Apples or yellow in the case of Yellow Delicious Apples.
Circuitry which has been designed for the sorting of fruits or vegetables generally provides some means for measuring the reflectance properties of the fruit or vegetable being examined. The reflectance of a surface is a measurement of the percentage of incident light reflected by it, and objects of a given color have different reflectances for light of different wavelengths. The relationship between reflectance and the illuminating wavelength for a type of fruit being sorted will produce a characteristic curve which can then be used in the design of apparatus and circuitry for color rating that fruit. That is to say, a fruit may be color classified by suitably measuring, describing, and classifying its reflectance curve, and fruit may be sorted into different grades by denoting the differences between the reflectance curves for the various grades and testing for these differences.
To achieve such a measurement, reflected light centered at two distinct wavelengths is measured. One of the selected wavelength bands will include light at frequencies wherein the variation of reflectance between distinct color grades of the fruit is at a maximum. The other band will comprise wavelengths where there is little or no variation in the reflectance between the different color grades. The determination of the color and individual fruit is then determined by observing the ratio of the values of the reflectance at the two different wavelengths. By using a ratio, the system automatically compensates for variations in factors unrelated to color such as strength of the incident light, size of the fruit, and partial obscuring of the viewing lenses.
The majority of prior art devices have utilized means for conveying the fruit past a color sorting head, the sorting head including means to illuminate the surface of the fruit and photodetectors to detect the intensity of light at various wavelengths reflected from the surface of the fruit. The observed light is split into two fractions centered at two different wavelengths of light, as described in the previous paragraph. The ratio between the intensities of the light received in the different bands is used as an indication of the color value of the fruit. Downstream from the sorting head are located one or more reject mechanisms which divert the selected fruit from the fruit carrying conveyor in accordance with its color value.
A critical factor in the success of any such color sorting apparatus is the design of the color sorting head. The color sorting head must be capable of viewing a substantial portion of the outer surface of each individual fruit and, at the same time, be simple in design so that it is rugged and economical. Various designs have been proposed, as described hereinafter, but none has been wholly successful.
One approach has been to surround the individual fruit with a plurality of light sources as the fruit is passed in front of two mirrors disposed on either side. Light reflected from the surface of the fruit is further reflected by each of the mirrors to a point above the fruit where the light beams are combined by a series of mirrors and lenses into a single beam which is then processed photoelectrically. Such an apparatus is disclosed in the patents issued to Thayer and to Roberts et al, U.S. Pat. Nos. 3,173,017 and 3,206,022, respectively. A second type of color sorting head is disclosed in the patent to Greenwood et al, U.S. Pat. Nos. 3,770,111. There, individual fruit are conveyed through the center of a fiber optic ring which detects light directed onto the fruit by a plurality of light sources and reflected therefrom. By using a fiber optic ring, this color sorting head is able to view substantially all of the surface of the fruit under inspection.
The use of separate sorting heads, each combining means to illuminate the fruit and means to observe the reflected light, to simultaneously view opposite sides of a piece of fruit is disclosed by U.S. Pat. No. 4,106,628 to Warkentin et al.
While the color sorting heads described hereinabove are functional, problems remain with the designs. First, the designs of Thayer, Roberts et al and Greenwood et al are intricate, requiring alignment of a number of mirrors and lenses relative to the frame of the conveyor. Such intricate systems are necessarily costly. A second problem arises in each of the above designs because hardware must be placed all around each fruit as it passes along the supporting conveyor. Placement of the hardware, in turn, requires that individual conveyor lines in multiconveyor systems be widely spaced, a requirement which increases the cost of the final system.